Several time codes have been established to provide time of day in serial format. One common code, the NASA 36 Bit Time Code, includes hours, minutes and seconds as well as day of the year, and a station identification. This code is transmitted by amplitude modulation of a 1000 hertz sine wave. The time code sequence repeats each second, using a pulse duration code. Binary 1's are pulses 6 milliseconds in length, binary 0's are pulses 2 milliseconds in length. These bits are shown as modulations three times the amplitude of the normal 1000 hertz sine wave.
In using this type time code, several problems have become evident. When the code is recorded on analog magnetic tape, some distortion of the AM envelope is experienced. This occasionally results in the improper decoding of the data bit. Allowing a margin of error can minimize this problem. Second, in the serial decoding of the time, the BCD time of day refers to the time epoch at the beginning of the 1 second frame. If display is made after decoding, the displayed number is 1 second behind. This can be corrected by adding 1 second to the received time code input. Third, the frame identification mark of NASA-36 bit code is a series of binary 1's. This series can occur in the station code location as well as the frame mark location. Expanding the frame mark definition to include a following zero puts the decoder 1 bit late, but removes the ambiguity.
The simplified time reader proposed in accordance with the present invention is intended to be simple, flexible and inexpensive. Parts used are all commercially available. Optional displays not required can simply be omitted. The basic consideration is simplicity, correction of the one second lag, availability of decoded day and station code, input variations of voltage (dB volts above and below 1 volt RMS) and capability of operation from one eighth to sixteen times normal code speed.
Prior art decoders have been more complex and expensive and dependent on frequency. Most time code readers simply demodulated the carrier-plus-pulse signal of the time code into its component parts, thereby retrieving the original pulse. The present invention operates normally at 1 KHz input frequency but operates as well between 125 Hz and 16 KHz. Where events are happening very quickly or very slowly, the reference time can be altered by varying the operating frequency; the information can be recorded and played back at a desired speed. One major reason for this flexibility is that the input part of the system is constructed, to a large extent, of passive elements (capacitors, resistors, etc.) with a minimal number of frequency-dependent, active elements (transistors, etc.).